1. Field of the Invention
Embodiments of the present invention relate to biphenyl derivatives and an organic electroluminescent device using the same. More particularly, various embodiments of the invention pertain to biphenyl derivatives having alkoxy group(s) and substituted or unsubstituted amino group(s), and an organic electroluminescent device using the same. The electroluminescent device offers improved efficiency and emissive characteristics.
2. Description of the Related Art
An organic electroluminescent (EL) device is an active drive type emission display device that operates under the principle that when current is applied to fluorescent or phosphorescent organic compound layers (hereinafter “organic layers”), electrons and holes are combined in the organic layers to then electroluminesce. Organic electroluminescent devices have various advantages including being lightweight, simple components, having a simplified fabrication process, and offering a wide range of colors with high luminescence. Also, organic EL devices can enable moving picture display perfectly with high color purity, and the devices have electrical properties suitable for portable electronic devices at low power consumption and low driving voltage.
Such organic EL devices typically can be classified into low molecular weight organic EL devices and polymer EL devices depending on their materials.
Low molecular weight organic EL devices have advantages including the simple and easy synthesis and purification to a high degree of emissive compounds, and color pixels of the three primary colors can easily be obtained. However, since organic layers typically are formed by vacuum deposition, low molecular weight organic EL devices are difficult to be suitably applied for formation of large-area layers, in which spin coating or ink-jet printing is generally employed.
In polymer EL devices, thin layers can be formed easily by spin coating or printing, so that the polymer EL devices can be fabricated in a simplified manner, and can be fabricated easily into a large-screen size at low costs. However, such polymer-based EL devices have lower emission efficiency than low molecular weight EL devices, and they have experienced shortened lifetime characteristics due to deterioration of emissive polymer. Since defects that promote deterioration in molecular chains are generated during synthesis of such polymer materials and impurities are difficult to refine, it is difficult to obtain high-purity materials.
To address the problems of polymer-based EL devices while having advantages of both polymers and low molecular weight materials, there is a demand for development of new materials. That is, development of high-purity, high-efficiency materials which have high purity, high efficiency and satisfactory reproducibility in synthesis, which can easily attain a reproducible molecular design, and which are suitably applicable for a large-screen size, is highly requested.
The electroluminescence mechanism of a general organic EL device will now be described. Holes are moved from an anode to an emissive layer via a hole transport layer, and electrons are moved from a cathode to the emissive layer via an electron transport layer. The electrons and holes meet in the emissive layer for recombination, forming excitons. The excitons are subjected to radiative decay, producing light having a wavelength corresponding to a band gap of an emissive layer forming material.
Materials for forming the emissive layer usually are classified into fluorescent materials using singlet excitons and phosphorescent materials using triplet excitons according to the electroluminescence mechanism. Conventional organic EL devices generally use fluorescent materials using singlet excitons. In this case, approximately 75% of the energy of the produced singlet excitons is not utilized at all. Thus, in the case of using fluorescent materials as materials for forming an emissive layer, that is, in the case of using the fluorescence mechanism originating from the singlet excited state, the maximum internal quantum efficiency is approximately 25%. Further, since the refractive index of a substrate material is affected by light extraction efficiency, the actual external quantum efficiency is further reduced, that is, at most 5%. As far as phosphorescence from a singlet excited state is utilized, a reduction in the external quantum efficiency unavoidably occurs when the injected holes and electrons recombine in the emissive layer. Thus, various entities have attempted for a long time to enhance the emission efficiency utilizing a triplet excited state with 75% efficiency. However, a transition from a triplet excited state to a singlet ground state is forbidden; it is generally a non-radiative transition and there are some difficulties in utilizing the same.
Recently, EL devices that utilize phosphorescence through triplet excitons have been developed. Phosphorescent materials can be prepared by doping various metal complexes, e.g., Ir or Pt, into low-molecular weight hosts or polymer hosts. Examples of the metal complexes include an iridium complex [Ir (ppy)3:tris(2-phenyl pyridine) iridium]. In the EL devices utilizing phosphorescent materials, efficiency, luminance, and lifetime characteristics thereof depend on the host material and the metal complex as a dopant. Therefore, the host material should be thermally, electrically stable.
Currently, 4,4′-N,N′-dicarbazole-biphenyl (CBP) is widely used as the host material. When an emissive layer is formed of CBP by vacuum deposition, an amorphous layer is homogenously formed. However, after performing vacuum deposition, the emissive layer may gradually lose homogeneity due to crystallization or coagulation, resulting in deterioration in emission efficiency and lifetime characteristics.
To manufacture highly efficient polymer EL devices that can overcome the problems, there is demand for development of new host materials having excellent crystal stability.